The corrosion of metals in liquid or gaseous media is an age-old problem. In the field of geological exploration in particular, the metal surfaces of the equipment used have to be protected against corrosion, for example under the effect of the mildly acidic or deaerated saline aqueous solutions which are used in the production and processing of petroleum and natural gas. Petroleum and natural gas and the water accompanying them contain corrosive constituents, for example CO.sub.2 or H.sub.2 S and salts, which lead to serious corrosion of metal surfaces. In addition, the working fluids, for example drilling muds, used in this field also contribute towards corrosion.
Accordingly, so-called corrosion inhibitors are used to provide protection against corrosion, being added to the liquids or gases which come into contact with the metal surfaces. The corrosion inhibitors either form a film on the metal surface or reduce the corrosion process by physicochemical reactions on the metal surface (cf. P. H. Ogden, Chemicals in the Oil Industry, The Royal Society of Chemistry, 1991, pages 21-22 and O. Lahodny-Sarc, Corrosion Inhibition in Oil and Gas Drilling and Production Operations, Eur. Fed. Corros., Publ. 1994, 11, pages 104-112).
Various substances, normally containing nitrogen, have already been proposed as corrosion inhibitors (O. Lahodny-Sarc, pages 112-113). Mitzlaff et al. (Werkstoff und Korrosion, 40, 629-634 (1989)) describe quaternary ammonium compounds as corrosion inhibitors for the production of petroleum and natural gas. Phillips et al. (Proceedings of the 8th European Symposium on Corrosion Inhibitors, Suppl. N. 10, 1995, 1213-1227) describe certain betaines, for example cocoamidopropyl compounds, for the same purpose. EP 320 769 A2 discloses ethoxylated quaternized ammonium compounds specifically for use in the w/o emulsions encountered in the production and processing of petroleum.
More recently, corrosion inhibitors have also had to satisfy more stringent requirements in regard to their biodegradability and aquatic toxicity. EP 651 074 describes N-ethoxyimidazolines substituted in the 2-position which not only have a favorable corrosion-inhibiting effect, they also show low aquatic toxicity (EC.sub.50 in Skeletonema costatum&lt;1 ppm).
Since it is precisely the chemicals used in the production of petroleum and natural gas which are having to meet increasingly more stringent environmental compatibility requirements, there is still a need to protect metal surfaces exposed to corrosive liquid or gaseous media against corrosion without using substances which have only limited environmental compatibility.
It has now been found that certain quaternized ammonium compounds which contain at least one ester group in the molecule have a favorable corrosion-inhibiting effect, are readily biodegradable and show low aquatic toxicity.